Horst and graben exist along mid-ocean ridges, where new crust is being made. Explain why.?

A horst is a block of rock which has risen between two faults. A graben is a block of rock that has sunk between two faults. So i need to know why they are found in mid-ocean ridges. This question is from Science Focus 3 by Kerry Whalley, Peter Roberson, Greg Rickard and Isabella Brown.Horst and graben exist along mid-ocean ridges, where new crust is being made. Explain why.?
A horst is not exactly a block that has risen between two faults. Actually, it is just a block that has not sunk, between two other blocks that have sunk (grabens). This kind of formation is typical of a spreading crust, where the tectonic plates are moving away from each other. That's exactly what happens in mid-ocean ridges. The spreading movement generates faults and makes some blocks to sink into mantle.